Optical discs, particularly, read-only optical discs, such as ROM discs, are used as package media all over the world since replica substrates of each disc can be mass-produced by plastic injection molding using a single stamper at low cost in a short time. For example, CDs (compact discs) and DVDs (digital versatile discs) are widespread as ROM discs on which information, such as a piece of music or video images, is recorded.
Related art is disclosed in Japanese Patent No. 3454410.